evaluation - Scheduled tasks and dynamic updating

Here is an example:

x = 0; Dynamic[x]

now please run:

SessionSubmit[x = 1; Pause[10];]

Problems:

• 
  

  Annoying one: Dynamic[x] is not updated before Pause is finished

  
  


• 
  

  Serious one: if Pause or whatever that follows x=1 is longer than DynamicEvaluationTimeout (6sec) then the FrontEnd will become unresponsive.

  
  

  This means that by doing a very standard thing and not requiring any fancy result you will get your FE stuck just because you modified a tracked variable within a scheduled task.

  
  

Diagnosis:

It looks like scheduled evaluations are or behave as wrapped with PreemptProtect.

One can see via LinkSnooper that soon after x=1 the Kernel tells the FrontEnd to update dynamic object. FrontEnd, using preemptive link, asks for the content of Dynamic[x] but the response is delayed till pause is finished. That delay is what causes problems.

One should note that in case x=1;Pause[10] evaluated as a normal code line it does not happen because preemptive link calculations can interrupt main link calculations. There is no documentation about what should happen when the long calculation is initiated from a scheduled task as opposed to a main link request.

Questions:

• 
  

  is that behavior expected? (not being able to interrupt a scheduled task)

  
  


• 
  
  

  is there a workaround? For simple cases one could do SessionSubmit[x = 1; SessionSubmit@Pause[10];] but that does not scale at all.

  
  


• 
  

  at the end I could live with waiting till whole task is finished but I can not accept that the FrontEnd stalls. To prevent it I'd need to make x=1 not to trigger updating at all till the scheduled evaluation is finished. That way FE won't ask back to early and won't get stuck.

  
  

Answer

• is that behavior expected? (not being able to interrupt a scheduled task)

Yes, as ihojnicki reminded us: sheduled tasks are preemptive.

• is there a workaround?

There is a workaround I am using for some time so I decided to share it.

The problem appears in a notebook based FrontEnd with Dynamic interactivity enabled, fortunately because the workaround requires being able to launch dialogs and use Dynamics.

The solution is the same as here 112332. Instead of a scheduled evaluation we schedule a launch of a dialog which will trigger a main link evaluation of a desired procedure.

Here is a function:

MainLinkSubmit::usage = "MainLinkKSubmit[procedure] allows you to submit a non preemptive call from a preemptive one "<>
    "(e.g. from scheduled task). Notebooks based front end environment is required.";

MainLinkSubmit // Attributes = {HoldAll};

MainLinkSubmit[procedure_] /; TrueQ @ $Notebooks := MessageDialog[
  Dynamic[
    NotebookClose[]; procedure

    , SynchronousUpdating->False
  ]
  , CellContext -> $Context
  , Visible     -> False
];

And here is an example:

x = 0; Dynamic[x]

SessionSubmit @ ScheduledTask[
  MainLinkSubmit[x = 1; Pause[1]; x = 2; Pause[1]; x = 3]
, {2}
]

FYI, this function is included in a prototype of my utilities package: https://github.com/kubaPod/Meh